The present invention relates to a pen core for a writing instrument, which has an ink conducting core for conducting ink to the writing member of the instrument.
Recently, a liquid-type writing instrument having an ink conducting core adapted to conduct ink to the writing member and a barrel with an ink reservoir which is fitted to the outer wall of a cylindrical pen core body has been extensively employed, for instance, as a plotter pen. In order to prevent the ink from dripping down the writing member upon a temperature change or an impact exerted thereon, the writing instrument employs a fountain pen type pen core which has a number of parallel, annular grooves, an ink groove crossing these grooves, and an air groove. Also, a spiral-type pen core has been used which has a spiral groove for communicating the ink reservoir with the writing end and the atmosphere. The parallel, annular grooves, or the spiral groove, are used as an ink pool to retain excess amounts of ink. The fountain pen type is disadvantageous in that, as an excess quantity of ink from the ink groove flows into the annular grooves simultaneously, such ink tends to collect in the annular grooves at the front ends thereof during the writing operation, flowing to the front end portion of the air groove communicated with the annular grooves and obstructing the air flow therein. Furthermore, the ink may flow out of the air groove if an impact is exerted on the pen core. Further, the spiral-type pen core is disadvantageous in that as the spiral groove is communicated with the air outlet at the front end and with the ink reservoir at the rear end, excess ink pooled temporarily is liable to flow down towards the front end of the pen core upon an impact applied thereto or by force of gravity during the writing operation, thus obstructing the air flow. Furthermore, the ink may flow out of the air groove if an impact is exerted on the pen core. Thus, in each of the above-described pen cores, the excess ink retained temporarily is liable to flow down to the front end of the pen core, even if the ambient temperature is constant and the pen is protected from impact forces. Therefore, ink tends to gradually collect near the front end of the pen core without completely returning to the ink reservoir with which the pen core is communicated at the rear end. As a result, the ink thus collected can obstruct the flow of air and drip out. Thus, such pen cores are not always satisfactory in performance.
In a writing instrument with a conventional pen core, an arcuate ink layer is formed by surface tension along the annular corner which is formed by the inner wall of the ink reservoir and the rear end face of the pen core body. When the level of ink in the ink reservoir becomes lower than the ink conducting core when the pen is held horizontally, the supply of ink to the ink conducting core is stopped. Thus, economical use of ink cannot be expected, and hence a writing instrument with the conventional pen core is not suitable as a liquid-type writing instrument which is installed, for instance, on a plotter in which writing operations must be carried out over long periods using only a small ink reservoir.